System and method of 3D print modeling utilizing a point cloud representation and generating a voxel representation of the point cloud

ABSTRACT

A method of 3D print modelling includes: generating a point cloud representation of a target virtual object for 3D printing, generating a voxel representation of the point cloud by adding a respective first voxel when a respective first point from the point cloud representation is located in the notional volume occupied by that first voxel, assigning a first colour associated with the first point to the faces of the first voxel, and if a second point from the point cloud representation is located within the volume of the first voxel, assigning a second colour associated with the second point to a respective face of the new voxel corresponding to the normal of the second point; and thickening the voxel representation of the point cloud by generating a duplicate voxel adjacent to the first voxel along a first axis passing through two opposing faces of the first voxel respectively having the first and second colours.

BACKGROUND

The present invention relates to a system and method of 3D printmodelling.

3D printing is a means of volumetric printing, where instead ofdepositing two-dimensional pixels on a flat surface, the printerdeposits three-dimensional voxels within a volume. There are numeroustechnologies to achieve this, including the use of lasers to melt andfuse materials such as metal powder in incremental layers to build astructure, or the use of a high-temperature print-head to deposit smalldroplets of plastic that cool and harden to build up a 3D model.

The design for such a 3D model is typically generated in acomputer-aided design program, in which a user defines surfaces andvolumes of a virtual model. A drawing list is then generated by theprogram specifying the order in which to fuse or deposit materials torender the virtual model using the 3D printer.

This approach has resulted in the creation of many aesthetically andfunctionally unique objects, some of which are difficult to make usingconventional processes. However the utility of 3D printers has still notbeen fully explored.

SUMMARY

The present invention aims to provide a new use for 3D printers.

In a first aspect, a method of 3D print modelling is provided.

In another aspect, a device arranged to generate a model for 3D printingis provided.

Further respective aspects and features of the invention are defined inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an entertainment device in accordancewith embodiments of the present invention.

FIG. 2A is schematic diagram of a side elevation of a 3D printer inaccordance with embodiments of the present invention.

FIG. 2B is schematic diagram of a plan view of a 3D printer inaccordance with embodiments of the present invention.

FIG. 3 is a schematic diagram of a videogame virtual environment inaccordance with embodiments of the present invention.

FIG. 4 is a schematic diagram of a projection in to a 3D model space inaccordance with embodiments of the present invention.

FIG. 5 is a schematic diagram of default virtual camera positions for avirtual photogrammetry process in accordance with embodiments of thepresent invention.

FIGS. 6A and 6B are illustrations of a 3D model generated by a virtualphotogrammetry process based upon renders from the default virtualcamera positions.

FIG. 7 is a flow diagram of a method of 3D print modelling in accordancewith embodiments of the present invention.

FIG. 8A is an image of a target object in accordance with embodiments ofthe present invention.

FIG. 8B is an image of a voxel model of the surface of the targetobject, in accordance with embodiments of the present invention.

FIG. 9 is a flow diagram of the method of 3D print modelling inaccordance with embodiments of the present invention.

FIGS. 10 A-E are schematic diagrams illustrating a process of generatinggeometry for 3D printing in accordance with embodiments of the presentinvention.

FIG. 11 is a schematic diagram of a 3D printable videogame character inaccordance with embodiments of the present invention.

FIG. 12 is a schematic diagram of a 3D printable videogame character inaccordance with embodiments of the present invention.

FIGS. 13A and B are illustrations of a zero-thickness virtual objectwith different colours or textures on each side, in accordance withembodiments of the present invention.

FIG. 14A is a schematic diagram of a voxel highlighting coloursassociated with respective faces of the voxel in accordance withembodiments of the present invention.

FIG. 14B is a schematic diagram illustrating successive colourassignments to faces of a voxel in accordance with embodiments of thepresent invention.

FIG. 15 is a schematic diagram of a voxel and a duplicate voxelhighlighting colours associated with respective faces of the voxels inaccordance with embodiments of the present invention.

FIGS. 16A and B are schematic diagrams illustrating a first thickeningtechnique in accordance with embodiments of the present invention.

FIGS. 17A to D are schematic diagrams illustrating a second thickeningtechnique in accordance with embodiments of the present invention.

FIG. 18 is a flow diagram of a voxel structure thickening method for amethod of 3D print modelling in accordance with embodiments of thepresent invention.

DETAILED DESCRIPTION

A system and method of 3D print modelling are disclosed. In thefollowing description, a number of specific details are presented inorder to provide a thorough understanding of the embodiments of thepresent invention. It will be apparent, however, to a person skilled inthe art that these specific details need not be employed to practice thepresent invention. Conversely, specific details known to the personskilled in the art are omitted for the purposes of clarity whereappropriate.

In an embodiment of the present invention, an entertainment device isoperably coupled to a 3D printer. The entertainment device is arrangedin operation to capture snapshots of videogame play for replication bythe 3D printer, as explained later herein.

Entertainment Device

An example of a suitable entertainment device is the Sony® PlayStation4® device.

FIG. 1 schematically illustrates the overall system architecture of theSony® PlayStation 4@ entertainment device. A system unit 10 is provided,with various peripheral devices connectable to the system unit.

The system unit 10 comprises an accelerated processing unit (APU) 20being a single chip that in turn comprises a central processing unit(CPU) 20A and a graphics processing unit (GPU) 20B. The APU 20 hasaccess to a random access memory (RAM) unit 22.

The APU 20 communicates with a bus 40, optionally via an I/O bridge 24,which may be a discreet component or part of the APU 20.

Connected to the bus 40 are data storage components such as a hard diskdrive 37, and a Blu-ray® drive 36 operable to access data on compatibleoptical discs 36A. Additionally the RAM unit 22 may communicate with thebus 40.

Optionally also connected to the bus 40 is an auxiliary processor 38.The auxiliary processor 38 may be provided to run or support theoperating system.

The system unit 10 communicates with peripheral devices as appropriatevia an audio/visual input port 31, an Ethernet® port 32, a Bluetooth®wireless link 33, a Wi-Fi® wireless link 34, or one or more universalserial bus (USB) ports 35. Audio and video may be output via an AVoutput 39, such as an HDMI port.

The peripheral devices may include a monoscopic or stereoscopic videocamera 41 such as the PlayStation Eye® or PS Camera®; wand-stylevideogame controllers 42 such as the PlayStation Move @ and conventionalhandheld videogame controllers 43 such as the DualShock 4 ®; portableentertainment devices 44 such as the PlayStation Portable® andPlayStation Vita®; a keyboard 45 and/or a mouse 46; a media controller47, for example in the form of a remote control; and a headset 48. Otherperipheral devices may similarly be considered such as a phone ortablet, printer, or a 3D printer (not shown).

The GPU 20B, optionally in conjunction with the CPU 20A, generates videoimages and audio for output via the AV output 39. Optionally the audiomay be generated in conjunction with or instead by an audio processor(not shown).

The video and optionally the audio may be presented to a television 51.Where supported by the television, the video may be stereoscopic. Theaudio may be presented to a home cinema system 52 in one of a number offormats such as stereo, 5.1 surround sound or 7.1 surround sound. Videoand audio may likewise be presented to a head mounted display unit 53worn by a user 60. The head mounted display unit may have integralheadphones, attachable headphones/earbuds, or rely on separate audiobeing supplied to the user.

In operation, the entertainment device defaults to an operating systemsuch as a variant of FreeBSD 9.0. The operating system may run on theCPU 20A, the auxiliary processor 38, or a mixture of the two. Theoperating system provides the user with a graphical user interface suchas the PlayStation Dynamic Menu. The menu allows the user to accessoperating system features and to select games and optionally othercontent.

3D Printer

As was noted previously, the entertainment device is operably coupled toa 3D printer.

It will be appreciated that there are numerous technologies for 3Dprinting that are typically differentiated by how layers are built up tocreate a model. One technology is known as selective laser sintering(SLS), where a layer of powder is selectively fused together by a laserto create solid regions; a new layer of powder is then placed on top andthe process repeats to build the model. Another technology is known asstereolithography, and uses a photo-reactive liquid in conjunction withone or more lasers to harden the liquid at defined points, therebybuilding the model, again typically in layers. Whilst both of thesetechniques are envisaged within the scope of embodiments of the presentinvention, they have the disadvantage of requiring both powerful lasersand large volumes of matrix material in the form of powder or liquidaround the model, which make them less practical for domestic use.Consequently a preferred technology is fused deposition modelling (FDM).This approach melts plastic in a printer head that moves over successivelayers of the model, depositing droplets of plastic at selectivepositions in each layer in a manner similar to the deposition ofdroplets of ink by an inkjet printer on a sheet of paper. This avoidsthe need for lasers or a surrounding matrix of the raw material used bythe model. Accordingly for the purposes of understanding an FDM 3Dprinter is briefly described herein with reference to FIGS. 2A and 2B.

FIG. 2A shows a side elevation of a simple FDM 3D printer 100, whilstFIG. 2B shows a plan view of the same FDM 3D printer. The printer 100comprises a base structure 110 that provides a working surface forassembly of the printed model and support struts 102 upon which aprinter frame 120 can be raised and lowered.

In an example mechanism, a motor 112Y is coupled to a drive bar 104comprising a screw thread; a printer frame 120 then comprises a couplingsection with a threaded hole and a smooth hole, the threaded hole beingengaged with the drive bar and the smooth hole being engaged with one ofthe support struts. When, under instruction from a printer driver, themotor 112Y turns the drive bar in a clockwise or anticlockwisedirection, the printer frame is moved up or down the drive bar andsupport strut (i.e. along the y-axis) and hence raises or lowers aprinter carriage 130 attached to it.

As can be seen from FIG. 2B, the printer frame 120 is typically mountedon four support struts 102. Optionally a second motor, drive bar andcoupling section may be provided at an opposing corner of the frame, toreduce strain on the frame and the motor.

The printer frame 120 supports the printer carriage 130. A motor 112X iscoupled to a drive band 122 that is fixed by fixing means 124 to theprinter carriage 130. When, under instruction from a printer driver, themotor 112X rotates the drive band clockwise or anticlockwise, theprinter carriage 130 is moved right or left along the printer frame 120(i.e. along the x-axis) and hence moves an attached printer mechanism140 laterally.

The printer carriage 130 supports the printer mechanism 140. A motor112Z is coupled to a drive band 132 that is fixed by fixing means 134 tothe printer mechanism 140. When, under instruction from a printerdriver, the motor 112Z rotates the drive band clockwise oranticlockwise, the printer mechanism 140 is moved in or out of a depthdirection (i.e. along the z-axis).

The printer mechanism 140 itself comprises heating means for heating theend of one or more plastic threads fed into the mechanism (not shown),or for heating grains of plastic powder obtained from a hopper on themechanism (not shown). The heating of the plastic or alternatively therelease of heated plastic is controlled by instruction from a printerdriver. A resulting bead or droplet of melted plastic is then depositedonto the working surface 110 of the printer or a partially built model(not shown) as applicable, through the printer head or nozzle 142.

In this way, under suitable instruction from a printer driver, theprinter head can be positioned anywhere within a working volume of the3D printer using motors 112X, Y, Z, and a droplet of plastic can bedeposited at that position, which then cools and hardens to form a voxelof a 3D model. Through successive movement of the printer head andselective melting or release of plastic droplets, a completed 3D modelcan thus be built from a plurality of such voxels.

Typically the printer driver itself is a software module in acomputer-aided design system that receives model geometry describing the3D model. The printer driver then generates thin slices of the 3D modelone voxel thick for each layer in the y direction, and determines the x,z coordinates for each voxel in that layer. The printer driver thenoutputs a sequence of instructions to the printer 100 to move theprinter head 142 to the respective x, z coordinate for each voxel inlayer y, where the printer mechanism is instructed to heat and/orrelease a droplet of plastic to form a voxel at that position. In thisway the digital 3D model is rebuilt as a physical model by the 3Dprinter.

In an embodiment of the present invention, the printer driver isincorporated into the videogame, or the operating system of theentertainment device, or a middleware library of support functions usedby either the videogame or the operating system.

Virtual Environment

Referring now to FIG. 3, a videogame running on the entertainment devicecomprises a virtual environment in which typically a plurality ofentities or environmental elements are dynamically viewed as the userchanges the position of viewpoint and as entities or elements of thegame enact scripted activities or react to the user's behaviour, or amixture of both.

In FIG. 3, a simple environment 200 is illustrated comprising a room 210with a door 212; on one wall there is mounted a candle in a candleholder214. In the room is the player's character 220 (here illustrated forexample by the fictitious game character Blobman).

The virtual environment is constructed in 3D from geometry data,typically in the form of polygons defining a surface of an object. Thesepolygons may be predefined for example in the case of static objects andbackground scenery, or may be warped/repositioned or procedurallygenerated in the case of mobile entities within the environment such asthe player's character. It will be appreciated that references herein to‘polygons’ encompasses preliminary geometrical features such asvertices, from which polygons are built, where these are used in thegraphics pipeline. Similarly, voxel rendering uses equivalent geometricfeatures to describe objects. Hence processes described as applying topolygons may be applied in whole or part to such other geometricalfeatures as appropriate.

The virtual environment is then prepared for presentation to the userfrom a selected viewpoint. Elements of the environment that have line ofsight to the user are patterned with texture information appropriate tothe object they represent, and the textured geometry is furtherprocessed for lighting effects, such as variations in brightness fromvirtual light sources, and bump mapping (or similar techniques such asdisplacement mapping or use of an isosurface) to simulate how thetexture should interact with such a virtual light source. Additionallyother effects such as volumetric fog and particle effects may beincluded.

The final rendered image is then presented to the user, typically via a2D or 3D television or via a head mounted display.

Often within such games, the user has an experience that they wish toshare with friends and/or the wider world. To this end, as notedpreviously an entertainment device such as the PlayStation 4 ® can havea ‘share’ button on its controller to facilitate a capture of the imagepresented to the user, which can then be uploaded to a sharing site suchas a social media site.

Printing a 3D model of the environment from a collection of images In anembodiment of the present invention, the user is provided with theoption to select a moment within the game from which to create a 3Dprinted model. In this way, the user can create tangible mementos oftheir in-game experiences.

In an embodiment of the present invention, when the entertainment devicereceives an input indicating that a user wishes to create a 3D print ofthe current scene, the game state is frozen (e.g. paused) so that it canbe analysed for 3D printing.

It will be appreciated that for a game, many visible features are notinherent in the in-game geometry or textures of a model alone. Forexample the in-game geometry, which may be a mix of polygons andskeletal models, may have physics effects such as positioning anddeformation applied from frame to frame. Other rendering effects likeprocedural effects and parametrised models may be done in shaders duringrendering. Meanwhile shaders may also visually process texturesaccording to bump maps and the like. Consequently often the finalscene—as viewed by the user—is only combined in one place, and in oneformat, when it is rendered for display on screen.

Consequently, in an embodiment of the present invention, a 3D model isconstructed for 3D printing using these rendered images in preference tothe potentially disparate internal in-game representations of thevirtual environment geometry. Such a technique may be referred to asvirtual photogrammetry. However, a single rendered image will typicallycomprise insufficient information to fully model the virtual environmentfor 3D printing.

As was noted previously herein, during normal play the virtualenvironment is rendered for a particular viewpoint. Furthermore toenable high frame rates and efficient processing, then as part of thisrendering process elements of the environment that are not visible fromthat particular viewpoint are culled early in the rendering process.

Consequently only the elements of the environment visible from theselected viewpoint are preserved and rendered. If one were to generate a3D printer model of the environment using this render, then all aspectsof the model that were not visible from that particular viewpoint wouldbe missing or would have to be filled in using some form ofinterpolation algorithm. Clearly this would give rise to unsatisfactoryresults when the real printed model was viewed from any other angle.

Accordingly, in an embodiment of the present invention, while the gamestate is frozen the entertainment device generates a plurality ofrendered images of the virtual environment from different viewpoints.

Preferably at least two opposing viewpoints are rendered, with the firstviewpoint typically being the one originally displayed to the user.Between the two images, this results in the rendering of most if not allof the elements of the environment culled in the original rendered view.

Optionally three viewpoints are rendered, preferably equally distributedon a plane, with the first viewpoint being the one originally displayedto the user and the plane being horizontally centred on the direction ofview of that first viewpoint. Again this results in the rendering ofmost if not all of the elements of the environment culled in theoriginal rendered view, but is likely to capture more elements of theenvironment that may have been occluded by objects both in front andbehind them and hence not rendered in either of the two viewpointsmentioned above.

More generally, as more viewpoints are distributed on the plane, fewerelements of the environment remain un-rendered. For example, theentertainment device may conduct a ‘fly-by’ within the environment,capturing N images, where N is for example between 2 and 360. The numberof images captured is a trade-off between fidelity of the eventual modeland the memory and processing capacity of the entertainment deviceneeded to analyse the captured images (as discussed later herein).

Optionally, one or more viewpoints looking down from above the virtualenvironment may also be rendered (either as individual images or as partof a flyby sequence) to provide additional detail for those elements ofthe environment that are substantially parallel to the previouslycaptured viewpoints on a single plane, such as the top surfaces of someenvironmental features.

In an embodiment of the present invention, metadata relating to theposition and viewing direction of the virtual camera representing theviewpoint for each image is also recorded and associated with therespective image.

It will be appreciated that the virtual environment may represent alarge area, whereas the 3D model will encompass a limited areadetermined by the size of models that can be generated by a particular3D printer and the scale at which the environment is printed. Preferablythe properties of the particular 3D printer are known if it is incommunication with the entertainment device (otherwise, a default modelsize may be assumed or the user may stipulate a size); similarly thescale at which the environment is printed may be selected by the user ormay be automatically determined with reference to a key character withinthe environment, such as the user's avatar. This avatar may be chosen tobe a particular size within the final 3D printer model (as anon-limiting example, 5 cm tall by default), and the extent of theenvironment to be printed at the same scale may thus be calculated.Alternatively, in a ‘character print’ mode, only a particular character,such as the user's avatar, may be printed, without surrounding in-gameenvironmental features. This may be of particular value for cases wherethe user is allowed to customise their own in-game character, andbecomes emotionally attached to it.

The equidistant distribution of two or more viewpoints may thus becentred on this key character, and may optionally use any in-game cameracontrol logic to determine the necessary direction of view needed tocentre the key character within each captured image.

Where the game presents a first-person view, then based upon the modelsize and an assumed or user-set scale, the centre point of the model canbe calculated and the viewpoints can be distributed around it.

Combining the above approaches, the entertainment device may captureimages by selecting sample points on a sphere of predetermined radius,and which is centred on that centre point. Optionally those samplepoints that are occluded by an environmental barrier (such as a point onthe sphere below the ground or inside a mountain) may either be skipped,or the radius of the sphere may be locally reduced until the surface isno longer occluded by the environmental barrier. Alternatively, elementsof the environment that are not part of a target object that is to be 3Dprinted may be omitted from the render to provide an unobstructed view(selection of a target object from the wider virtual environment isdescribed later herein). The sampling scheme may initially selectviewpoints on the above mentioned plane comprising the originalviewpoint and centre point and parallel to the horizontal axis of theoriginal viewpoint, before optionally selecting one or more viewpointson the sphere above this plane, and optionally one or more viewpoints onthe sphere below this plane. The predetermined radius may be equal tothe distance between the original viewpoint and the determined centrepoint of the model, to maintain consistency with the original capturedimage, or the original image and the subsequent additional image(s) maybe re-rendered at a different effective radius, for example a minimumradius at which the field of view of the image encompasses the groundarea that will be included in the 3D printed model. Notably,traditionally games use lower-fidelity models at greater draw distancesto simplify the rendering process. Therefore optionally the radius maybe constrained to a maximum distance equal to a rendering distance atwhich the game would select a lower-fidelity model of a key element ofthe image (such as the user's avatar). Further optionally, this may beoverridden by a user for aesthetic purposes or because they wish toensure that a particular environmental feature or combination offeatures is included in the final model. This may be of particular valuefor cases where the user is allowed to customise the environment, forexample by creating so-called ‘mods’.

In any event, the result is a set of images capturing two or morecomplimentary viewpoints of the virtual environment for a given gamestate.

It will be appreciated that the above image capture procedure may becontrolled by the entertainment device. However, alternatively or inaddition the user may capture images of the same scene from one or moredifferent viewpoints by selecting these viewpoints themselves and usinga conventional image capture process. These viewpoints may notcorrespond to the preferred distributions discussed previously herein.It will similarly be appreciated that images of the same scene fromdifferent viewpoints can be captured by different users at differenttimes on different entertainment devices; providing a user has access toa pooled set of images (for example if they are posted to an onlineforum, or are stills extracted from a ‘fly-by’ video that moves orchanges viewpoints, such as may be included in a trailer video for thevideogame) then an equivalent set of two or more complementaryviewpoints of the virtual environment may be obtained.

Given these images and optionally associated metadata relating to theviewpoint position and direction, an entertainment device can go on toanalyse these images to generate 3D model data.

Several analysis techniques may be used, optionally in a complementarymanner.

Silhouette analysis uses the edges of objects within the captured imagesto estimate the object's profile local to that edge. The object profilecan then be extruded orthogonal to the profile for each viewpoint untilit intersects with another extruded profile (typically extruded fromanother viewpoint), to create an approximation of the object's shape. Itwill be appreciated that as the number of viewpoints increases accordingto the scheme described previously herein, each extrusion will subtend asmaller angle around the object, resulting in a more accurate overallmodel of the object.

Stereoscopic analysis uses the relative displacement of the same objectswithin two overlapping viewpoints to calculate their distance from theor each viewpoint. From this information, a depth map can be constructedthat is indicative of the 3D shape of the object in the region visibleto both viewpoints. Hence again where more viewpoints are available,successive pairs of viewpoints can be used to build a map of the surfaceof a viewed object.

In either case, where there is no viewpoint information, this may beestimated by correlating environmental features between images, andselecting viewpoint positions and directions that provide the bestcorrelation for the relative positions of these features.

It will be appreciated that silhouette analysis and stereoscopicanalysis can be used to complement each other. For example the twotechniques can be used to detect errors in each other's models; wherethe two techniques generate estimates of an object surface that differby a threshold amount, the estimate that most closely matches aninterpolation between nearby estimates of the object surface from thetwo techniques that are within a threshold agreement may be used, withthe other estimate being discarded. Optionally, such areas of the modelmay also be highlighted for inspection and/or editing by the user priorto printing, as discussed later herein.

The above techniques are particularly useful where the image data isobtained from third parties (for example from screenshots on a forum),and there is no additional metadata available.

However, in the case of images generated and captured by theentertainment device, it is preferable to also capture associated depthinformation generated by the entertainment device itself. As was notedpreviously, during a conventional render of a viewpoint, elements of theenvironment that are occluded from the rendered view are culled. Thisocclusion is determined, at least in part, by the relative distance ofobjects in the environment from the virtual camera; for example objectsthat are behind other objects are deemed to be occluded and thus culled.These distances are calculated by the entertainment device for thispurpose. This means that there is an accurate and per-pixel (or evensub-pixel) resolution distance or ‘depth’ map available for eachcaptured image.

In an embodiment of the present invention, this depth map is alsocaptured and associated with each image captured by the entertainmentdevice for use in the generation of the 3D model. In principle, thisinformation can be obtained in a manner that is transparent to theoperation of the renderer by copying the so-called z-buffer used whengenerating the image. Consequently the image capture process does notrequire modification of a game rendering engine.

More generally, it will be appreciated that any suitable technique foridentifying the surface position of a rendered pixel within eachcaptured image can be made with respect to a 3D origin (or example anorigin within the game world or an origin for the render of the targetobject in isolation), thereby generating a consistent representation ofthe pixel over potentially multiple renders.

Referring now to FIG. 4, given information regarding the position A andviewpoint direction B of each image, together with the associateddistance information z, the pixels of each image can be projected torespective points within a shared virtual modelling space 300, forexample based on a common origin. In effect, each pixel is displacedfrom the position A of the image viewpoint by amount x,y,z in aco-ordinate scheme local to the image (but which may be rotated to aco-ordinate scheme common to the shared virtual modelling space, usingknown techniques). Here, x and y are the pixel co-ordinates in thecaptured image (represented by notional screen C), and z is theassociated depth or distance value for that pixel from the cameraposition A. Only one column of pixels (i.e. for a single value of x)have been projected in FIG. 4 for clarity, but it will be appreciatedthat any number of pixels of the image can be projected in this manner;for example, those corresponding to the target object.

The result is that pixels from each image will be positioned within theshared virtual modelling space at their correct position in 3D on thesurface of a virtual object in the scene, as illustrated in FIG. 4 usingthe example of Blobman. As each image is projected into the sharedvirtual modelling space, more of the surface of each object in the scenewill be ‘painted-in’ in this manner. Where two images project the samepixel onto the same point in 3D space, the second projected pixel may bediscarded or may replace the first pixel, or an average value may begenerated.

It will be appreciated that pixels are two dimensional objects.Consequently in an embodiment of the present invention, when a pixelfrom an image is projected to a position xi, yj, z_(k) in the sharedvirtual modelling space, in fact a voxel (a typically cubic3-dimensional pixel) is created at that position, and the colourinformation associated with the projected pixel is used as the colourinformation for the voxel.

The effect is that a shell of voxels representing the visible surface ofthe virtual environment is built up by projecting the colour data of theimage pixels in each image to x,y,z positions in the shared virtualmodelling space.

It will be appreciated that instead of projecting plural images into ashared modelling space using the z-buffer depth information from theentertainment device, this depth information may be estimated using thepreviously discussed techniques of stereoscopic depth estimation and/orsilhouette analysis and surface extrusion. Hence these techniques mayalso be used to drive the assembly of a voxel shell within a sharedmodel space in the above described manner.

In any event, once all the images have been used, the resulting shell ofvoxels can be analysed for integrity. Any gaps in a surface (optionallybelow a threshold size) may be filled in by interpolation from thesurrounding surface.

Having generated an estimate of the 3D shape of the environment in theform of the voxel shell, a 3D model can be generated for printing.

Determining and Improving Model Integrity Referring now to FIG. 5, in anembodiment of the present invention a target object (such as theillustrated character 520) is rendered from a plurality of viewpoints.Whilst previously it was suggested that at a minimum two images may beused, or only a few, in practice considerably more may be used in orderto provide good coverage of the target object(s). Hence in FIG. 5, as anon-limiting example eighty renders are generated for each of threecircular orbits of the character by a virtual camera, each orbit beingequidistant about a circle orthogonal to the orbits themselves. It willbe appreciated that the number of orbits and the number of renders perorbit are purely exemplary and any suitable combination of renders andorbits (or partial orbits, e.g. semicircular above a base height of thetarget object) may be considered, or distributions other than orbits(such as a distribution of camera positions over the effective surfaceof a sphere, either uniform or non-uniform, for example with additionalcamera positions near the position of the in-game camera when theoriginal indication to capture the object was made). At the same time,for each render optionally depth (distance) data for each rendered pixelfrom the virtual camera position is also recorded, so that the positionof each rendered pixel in the rendering space can be determined from therender, the depth information, and the camera position.

It will be appreciated that any scheme that results in sufficientviewpoints being provided may be appropriate; hence as an alternative tothree orbiting virtual cameras, a constellation of (for example 80)stationary virtual cameras may be used. Similarly in either case thetarget object may be static or may itself be rotated in one or moreaxes, for example to provide each of a smaller constellation of cameraswith several neighbouring views, effectively compensating for missingcameras at different viewpoints in a larger constellation.

The resulting N images (in this example 240) are provided as input to aknown photogrammetry technique, for example as described previously.

An example output of this technique is shown in FIGS. 6A and 6B. FIG. 6Ashows a model of the character derived using virtual photogrammetry, andsuperficially it appears to be accurate. However, as can be seen fromFIG. 6B, as highlighted by the oval ring, regions of the character'storso occluded primarily by the gun have not been modelled accurately,causing regions of the model to be fused or distorted.

Clearly a user having printed such a model would be disappointed withthe quality of the result.

Accordingly, and referring now also to FIG. 7, in an embodiment of thepresent invention a method of 3D print modelling comprises in a firststep s710, generating a voxel representation of the surface of a targetvirtual object.

It will be appreciated that having frozen the virtual environment/targetobject for the purpose of multiple renders, the actual object modelremains static during the virtual photogrammetry process. Consequentlyit is possible to generate a separate and persistent representation ofthe model to track the progress of data gathering by each successiverender. Referring now to FIGS. 8A and 8B, for the example case of asamurai character as rendered in FIG. 8A, a representation of thatcharacter can be generated as shown in FIG. 8B (in this case in aslightly different pose, about to swing an axe).

The separate and persistent representation is a voxel model of thesurface of the target object(s) to be printed. As can be seen from acomparison of FIGS. 8A and 8B, typically the voxel resolution is lowerthan the render resolution, so that plural pixels in a rendered surfacewill map to a corresponding voxel in the voxel model.

The voxel resolution may optionally be selected to correspond to thevoxel resolution of the 3D printer that is to be used. Hence for exampleif the 3D printer has a voxel resolution of 1 mm³, and the printed modelwill be 10 cm high, then the voxel model will be 100 voxels high. Itwill be appreciated that whilst it is preferable to use a voxelresolution at least the same or higher than the 3D printer, this is notessential.

The voxel model can be generated using model data extracted during therendering process, or can be rendered separately using the same targetobject model data.

In a second step s720, a counter is associated with each voxel. Thiscounter is used to tally how many rendered images include a pixel fromthe portion of the target object(s) corresponding to that voxel, as willbe explained later herein. Hence the voxels in conjunction with theircounters form a voxel histogram of how many times each surface regioncorresponding to a voxel gets rendered during the photogrammetryprocess. This counter may thus be thought of as the main histogramcounter for a voxel.

In a third step s730, the target virtual object(s) are rendered from aplurality of viewpoints as described previously, for example withreference to FIG. 5.

Then for each rendered viewpoint, in a fourth step s740, the mainhistogram counter associated with a voxel is incremented if a firstpixel on a portion of the object surface corresponding to that voxel isrendered.

As was noted previously, typically the voxel resolution is lower thanthe rendered pixel resolution, meaning that potentially multiple (andvariable) numbers of pixels may map to a corresponding voxel in anygiven render. Consequently the total number of pixels corresponding to avoxel in a render would be uninformative, and this would be compoundedif the count accumulated over multiple renders.

Rather, it can be assumed that the first pixel in the render that mapsback to a voxel can be taken as indicative that the region of the modelcorresponding to the voxel is appropriately visibly represented in thatrender.

A simple way to implement this check approach is to further associate aflag with each voxel, which is reset to a first state (e.g. to inactive,low, or 0, depending on convention) for each render. Subsequently, thecounter of a voxel is only incremented if the previously describedcondition of rendering a pixel on a portion of the object surfacecorresponding to that voxel is met, together with the condition that itsassociated flag is in the first state. Then, if the main histogramcounter is incremented, the flag is also switched to a second state(e.g. active, high, or 1, depending on convention).

By flipping the flag to a different state if the main histogram counteris incremented, and only incrementing that counter if the flag is in thedefault initial state, then the main histogram counter can only beincremented once per render, i.e. in response the first rendered pixeldetected to be in the region of the target object(s)' surfacecorresponding to that voxel.

It will be appreciated that in principle different counting strategiescould be used in addition to or instead of a flag scheme. For exampleoptionally a parallel counter could be provided for each voxel thatcounts up to M (where for example, M is a number corresponding to afraction of the pixels that in principle map to a voxel) when a pixelcorresponding to that voxel appears in a render.

Hence for example in the previous case of 1 mm³ voxels for a 10 cmmodel, resulting in a voxel representation 100 voxels high, then if therendered object is 1000 pixels high one can infer that potentially 100pixels (10×10 pixels) could map onto a voxel, for example if viewed faceon and without occlusion.

Hence for ½ of the pixels, the parallel counter would count to M=50.Meanwhile for 1/25 of the pixels, the parallel counter would count toM=4. In this case, the main histogram counter may only increment if theparallel counter reached the selected M threshold value. This wouldindicate that a non-trivial proportion of the pixels corresponding tothat voxel had been rendered in a given view. The proportion may forexample by anywhere in the range 100% to 1%, but more typically would bein the range 50%-1%, more typically still in the range 25%-2%, moretypically still in the range 15%-3%, and more typically still in therange 10%-4%.

Similarly optionally, a parallel counter may be provided that counts upto P, where P is again a proportion of the possible pixels that may berendered. At the end of the analysis of a given render, the final valuein the parallel counter is added to the main histogram counter. Thiswould enable a reflection of the different amounts of pixels beingrendered for each voxel in a view, whilst limiting the degree ofdisparity between histogram values (continuing with the above example,if the fraction was 1/10, so that P=10, then a voxel that had 100corresponding pixels rendered would only add 10 to its histogram, whilsta voxel that had 5 corresponding pixels rendered would add all 5 to itshistogram. Over time it would still be clear what areas of the modelwere better represented than others, but without value disparitiespotentially amounting to plural orders of magnitude, which could makesubsequent processing difficult.

Hence in this case, the method would comprise associating each voxelwith a second counter that is limited to a predetermined maximum countvalue, and the step of incrementing the main histogram counter wouldcomprise incrementing the second counter associated with a voxel,subject to the predetermined maximum count value, if a pixel on aportion of the object surface corresponding to that voxel is rendered,and then subsequently incrementing the main histogram counter by thefinal value of the second counter.

Referring again to FIGS. 5 and 8B, once all the renders at thepredetermined camera positions have been completed and the relevant datafor photogrammetry has been accumulated, the main histogram counters ofthe separate voxel model will have values indicative of how often (oroptionally, indicative of how completely) the corresponding regions ofthe target object were rendered over all the different camera positions.

Then, referring now to FIG. 9, in an embodiment of the presentinvention, the method of 3D print modelling proceeds as follows for eachvoxel:

In a fifth step s950, the counter is compared with a predeterminedthreshold value to detect whether it is below that value. If it is abovethe threshold value, then it is assumed that the region of the targetobject(s) corresponding to that voxel has been imaged sufficiently oftento provide a good input to the photogrammetry process, which can proceedas described previously in order to generate a 3D surface geometry (andoptionally but preferably also associated textures). The generate 3Dsurface geometry may be in the form of polygons, a point cloud, a highresolution voxel shell or any suitable representation for subsequentprocessing as describer later herein.

However, if the counter is below the threshold value, then it is assumedthat the region of the target object(s) corresponding to that voxel hasnot been imaged sufficiently often to provide a good input to thephotogrammetry process, and therefore at least one additional render isneeded. For example Q-R renders may be sought, where Q is thepredetermined threshold value and R is the actual value of the mainhistogram counter for that voxel. Alternatively, only one additionalrender may be sought.

To obtain this render, in a sixth step s960 a plurality of lines isprojected from the position of the voxel (for example 16, 32, 64, 128,256 lines equidistantly radiating from the centre or surface of thevoxel). These lines can be thought of as candidate directions of viewfor a virtual camera that are guaranteed to intersect with the region ofthe target object(s) corresponding to the voxel (when looking back downthe lines toward the voxel position).

Then, a seventh step s970 comprises detecting if any lines reach apredetermined distance without occlusion from other voxels. For examplewith reference to FIG. 5, it can be detected whether any lines reach thesurface of the sphere upon which the virtual camera is positionedwithout interruption/occlusion by other voxels.

It will be appreciated that the point of intersection of any line withthat sphere represents a position at with the virtual camera may beplaced in order to obtain a render that includes at least some of theregion of the target object(s) corresponding to the voxel, if the cameralooks along an equivalent line.

Hence in an eighth step s980, at least one such line (as transposed tothe render space, if the render space uses different co-ordinates) isselected as the optical axis for a further render of the target virtualobject(s), and in step s990 a render of the target object(s) along theor each selected optical axis is then generated.

Where a plurality of lines reach the predetermined distance withoutinterruption, then optionally all of them may be used in this manner, ora predetermined number of them, or Q-R of them, or just one. Where asubset of available lines are used, optionally the lines are chosen tomaximise the distance between the selected lines on the camera sphere,so as to obtain diverse viewpoints of the relevant region of the targetmodel(s).

In this way, additional renders for capturing views of under-representedregions of the target model can be identified and obtained to improvethe required coverage of the model for the purposes of photogrammetry.

Hence after his process has been completed for any voxels whose count isbelow the predetermined threshold, again the renders can provide a goodinput to the photogrammetry process, which can proceed as describedpreviously in order to generate a 3D surface geometry (and optionallybut preferably also associated textures). Again, the generated 3Dsurface geometry may be in the form of polygons, a point cloud, a highresolution voxel shell or any suitable representation for subsequentprocessing as describer later herein.

Hence whether additional renders are required or not, the direct productof the photogrammetry process is the 3D surface geometry of the targetobject (and optionally any associated textures. This 3D surface geometry(and optional textures) may be exported to a separate device formodification as described later herein, or saved, shared to a local orremote, private or public repository.

Meanwhile, if for a given voxel no lines reach the predetermineddistance without occlusion from other voxels, then optionally the numberof lines may be increased (for example from 128 to 256 lines), thusincreasing the line density and the opportunity for a line to have anuninterrupted route. Such a process may continue until a maximum numberof lines is reached (e.g. 512).

If no lines reach the predetermined distance without occlusion(optionally after increasing the number of lines) then the method maycomprise a step of issuing a warning to a user that the resultingphotogrammetry will be based on an incomplete view of the targetobject(s), and hence that a 3D printed model based on virtualphotogrammetry of the rendered object may contain errors. The method maythen terminate, offer the user the option of terminating, offer the userthe option of proceeding, or proceed.

In particular, where the number of lines reached a maximum numberwithout any line reaching the predetermined distance, this implies thatthe view of the model region corresponding to the voxel is very limited,and so any resulting distortion or error in the model in that region mayhave a minimal aesthetic impact.

Optionally, the user may be provided with one or more renders of themodel overlaid with a so-called ‘heat-map’, which colour codes regionsof the model based upon the histogram counter values of thecorresponding voxel. This provides the user with a simple indication ofthe quality of the source data (e.g. the number of renders) for anygiven region of the render.

Similarly, after photogrammetry is complete, the resulting model may berendered for the user, such that for example the user can control theviewpoint and inspect the model. Again optionally the resulting modelmay have a colour heat-map overlay provided that indicate to the userthe quality of the source data (e.g. the number of renders) for anygiven region of the resulting model. This may assist the user in moreclosely inspecting regions of the model that may comprises errors,before committing to 3D printing the model.

Modifying the Model

It will be appreciated that the 3D surface geometry generated by thephotogrammetry process, whether expressed in polygons, or as a pointcloud or as a high resolution voxel shell, will have a negligiblethickness corresponding either to a notional zero thickness, or a onepixel thickness or a one (high resolution) voxel thickness. In anyevent, when translated to a 3D printer, this is likely to be interpretedas one print voxel thick (e.g. using the above example, 1 mm thick).This is unlikely to be adequate in practice for any given model.

Accordingly, and treating the 3D surface geometry now as a shell ofvoxels to be printed, the shell can be extruded to a predeterminedthickness. The extrusion may be performed by adding adjacent voxels toeach surface voxel on the side opposite to the side(s) from which theywere projected into the shared virtual modelling space (i.e. on theinside or underside of the shell).

The number of voxels added may be the lesser of Y voxels or the numberneeded to reach an opposite existing surface of an object. Y may bechosen to result in a thickness responsive to the weight and strength ofthe material being used to print the model.

Similarly, the lowest point within the (now extruded) model can bedetermined. At least two additional points, typically on the undersideof the shell, can be extruded further to match the lowest point, thusproviding at least three points of support for the physical model.Preferably, these points of support are distributed around a calculatedcentre of mass of the model.

In this way, a 3D model of the virtual environment can be constructedfrom plural in-game images that will stay up when physically printed andis viewable from a plurality of angles.

The model is then sent to a 3D printer driver, which slices the modelinto layers from the bottom up. These layers are then successivelyprinted by the 3D printer as described previously.

As described above, photogrammetry of at least part of the virtualenvironment can be used to generate a voxel shell that can be used asthe basis for 3D printed model. Some environments are more suited tothis 3D printing process than others. For example, the simpleblock-based environment within Minecraft® will lend itself very well tobeing rendered by a 3D printer.

However, this is the exception rather than the rule. A particular appealof video games is their ability to present environments and charactersthat are not bound by the normal rules of physics. In particular,objects may exist in a predetermined relationship to each other withouthaving physical connections (as exemplified by the character ‘Blobman’in FIG. 3, whose hands and feet are not physically attached to his torsoin this figure), whilst other objects may be defined only in twodimensions within the three-dimensional environment, such as curtains,capes and in many cases environmental components such as walls. This isbecause physical strength is not a requirement of the virtualenvironment, where program rules prevent movement beyond certainboundaries, and the walls are simply decorative surfaces for texturesdemarcating those boundaries.

Hence a virtual environment may comprise several bounding surfaces thathave zero thickness, upon which are placed physical objects, some ofwhich may additionally be unstable or unsupported if physically printed.An example of such bounding surfaces may be the walls of the room 210 inthe virtual environment 200 of FIG. 3.

It may not be feasible to faithfully print such a virtual environmentusing a 3D printer.

Accordingly, in an embodiment of the present invention, the voxel shelldescribed previously is modified to take account of physicalrequirements of the 3D printed model.

As noted previously, the shell of voxels can be extruded to be M voxelsthick, to provide some physical strength. This also addresses theproblem of 2D objects within the environment, as the shell of voxelscorresponding to these objects are extruded in the same as any otherelement of the shell of voxels.

However for some elements of the environment, this may not providesufficient physical support, or in the case of aerial objects in theenvironment, support may be missing entirely.

Procedural Supplementary Geometry

Accordingly, procedurally defined modifications to the voxel shell maybe provided for the purposes of 3D printing. These proceduralmodifications provide structural strength and/or physical support forsome elements of the virtual environment that it would not otherwise befeasible to replicate in a 3D printed model.

Hereinafter references to ‘printer geometry’ refers the voxels createdto define the 3D model for printing.

Procedurally generated or modified printer geometry may be generated bythe entertainment device once the initial voxel shell has beengenerated.

The voxel shell is analysed to detect whether additional geometry isrequired, according to several basic rules.

One analysis step is to detect whether there are unsupported objectswithin the environment.

If so, then these may be made subject to a de minimis size threshold sothat objects below the threshold, such as for example snowflakes,bullets etc., are ignored and subsequently not retained within the 3Dmodel. The associated voxels in the voxel shell may be deleted.Optionally this step may be implemented in a modified rendering processso that these items are never rendered in the first place whencollecting images for the model, making the analysis of the imagessimpler.

Meanwhile for objects that meet the threshold, printer geometry for asupport structure such as connecting rod may be procedurally generatedbetween the object and a nearby anchor point, the anchor point being apart of the environment ultimately connected to the ground. The grounditself can be assumed to be connected to the lowest point of the voxelshell. Optionally the analysis can proceed from the lowest part of thevoxel shell upwards so that unsupported objects can be sequentiallysupported, enabling them to support subsequent objects in turn.

Consequently where a video game character is leaping into the air forexample, a supporting rod will be formed between the base of their footand the ground by creating a cylinder of voxels, for example M voxelsthick by default.

However, the thickness of the rod may also be procedurally determinedaccording to basic rules. For a given type of printer, the weight byvolume of common 3D printer materials and their compression and bendingstrengths will be known. In addition, the expected size of the printedmodel will be known. Accordingly, the entertainment device can estimatethe weight of unsupported objects, and calculate the thickness ofsupporting rod needed to adequately support the object within the model.In particular where a connecting rod is partly or wholly lateral andhence may bend, the moment of the unsupported object is a function ofits weight multiplied by its distance from the current position alongthe rod. Furthermore when constructing a connecting rod for such anobject, the weight of the connecting rod itself may be a significantproportion of the weight. Consequently the rod may be thicker closer tothe anchor point and taper towards the unsupported object as respectiveparts of the rod support a greater weight at the anchor point than atthe unsupported object.

In another analysis step, the above principle can be applied to otherparts of the voxel shell; for example a virtual model desk within a gamemay have legs that are too thin to support the total mass of the desksurface and any objects on the desk. For example, if the desk has acharacter standing on it, but the desk legs are only a few voxels thick,they may not be strong enough to support the physical model. Accordinglyfeatures of the voxel shell can be evaluated to determine the totalweight they will be supporting in the model, and where this exceeds asafe margin for the cross sectional area of the voxel shell supportingthis weight, this may be thickened by scaling up its cross-section andfilling it with voxels. The principle may also for example be applied totrees within the environment, where the cumulative weight of branchesand the trunk are used to locally modify the geometry of the branches ortrunk to make their cross-section larger to accommodate the weight, inthose circumstances where the existing geometry is locally insufficientto accommodate the weight when rendered as a 3D model using theparticular materials associated with the 3D printer.

Subsequently, the centre of gravity of the adjusted model may becalculated to detect if it would fall over. If so, then either a supportmay be moved to restore stability, or optionally the voxel shell may bethickened in one or more selected places to alter the centre of gravityback to a stable position.

As noted previously, two-dimensional components of the environment areextruded along with other parts of the initial voxel shell to create athree-dimensional object with at least a minimum thickness. However,this extrusion process can be further modified as described below.

Referring now to FIGS. 10A-E, it will be seen that FIG. 10A represents atwo-dimensional wall from the environment 200, upon which a candle andcandlestick are placed within the game. FIG. 10B then illustrates anextrusion process to extend the wall in a direction normal to the wallplane and opposite to the side from which the image was projected, toprocedurally generate geometry describing a wall with a predeterminedthickness, for example of Y voxels. However as can be seen in FIG. 10C,the cumulative weight of the physically printed wall will changeaccording to the position on the wall, and also encounters a step changefor parts of the wall additionally supporting the candle andcandlestick. Accordingly, a wall of constant thickness may be unsuitableas the thickness of the wall near the base may be insufficient toadequately support the cumulative weight of the model.

Accordingly, with reference to FIGS. 10D-E, then starting with a minimumpreferred thickness of extrusion at the top of the wall, the thicknessof the extrusion increases as a function of cumulative weight, resultingin a step change in thickness at the point of attachment of thecandlestick to the wall to provide additional support. It will beappreciated that procedural rules may thicken a region around suchpoints of attachment, such that the wall is thickened slightly beforethe load of the candlestick is applied to the wall, as illustrated inFIGS. 10D-E. In addition, the procedural rules may accommodate theadditional mass of the supportive thickening itself when extruding lowerportions of the wall. Finally, the load imposed by the candlestick andthe supporting region may be distributed laterally as well asvertically, so that the thickened supporting region splays sideways asit propagates down the wall, as shown in FIG. 10E.

The procedural rules may also interact with additional informationprovided by a game developer, to assist in generating desirablesupplementary geometry automatically.

For example, the previously noted rule that a connecting rod isprocedurally generated between an unsupported object and the nearestanchor point, where the anchor point is a part of the environmentultimately connected to the ground, may result in 3D model of Blobman220′ as illustrated in FIG. 11. It will be appreciated that when thecharacter's hands are analysed, it is likely that they are closer to thecharacter's feet than to the character's torso, and possibly also thatthe character's torso will not yet be connected to an anchor structureitself. Accordingly the procedural rules may attach the character's handto the only grounded anchor points available, which are the feet;subsequently the torso may be anchored to the closest single anchorpoint, which is now one of the hands. The result is a structure that,whilst stable, does not conform anatomically to what the user mightexpect.

Accordingly, game developers (or users) may identify preferred points ofconnection between elements of the environment in order to guide theprocedural generation of connecting rods or other connecting/supportstructures. These points of connection may be defined by use of areserved value within one or more colour channels, enabling points ofconnection to be identified by use of a specifically colouredidentifying pixel within an image; for example if the value 128 wasreserved on the blue channel to denote a connecting point, then this canbe used in the example of the Blobman character 220″ of FIG. 12 tospecify where preferred connecting points exist on the model. Thesereserved values are then used within the voxel shell, creatingidentifiable anchor points for use with the procedural rules. Forexample, matching values in the other two colour channels of theidentifying pixel can be used to indicate paired connecting points, sothat red and green values in the identifying pixel could be the same forthe hand and the torso, causing these to be identified as sections to beconnected together. In this way, artistic control of the placement ofconnecting structures can be maintained by the developers without theneed for separate metadata. In the case of Blobman, this could be usedto ensure an anatomically acceptable set of connecting rods, as can beseen in FIG. 7. Meanwhile, the rules for procedural generation of theconnecting/support structure will still determine the requiredcross-section needed to adequately support the printed structure.

It will also be appreciated that for elements of the supplementaryprinter geometry not found in the rendered game (such as the connectingrods in the ‘Blobman’ example above), textures (colours) may beextrapolated from the existing voxel shell, or a no colour may bespecified. In the case that colours are extrapolated, optionally thecolours may be extrapolated from the portion of the structure supportingan element against gravity (e.g. the ground, or in the case of Blobman,from the feet to the torso, and from the torso to the hands and head,respectively), as this is likely to create the most unobtrusivesupporting structure within the model. Alternatively in the case ofusing no texture, the bare plastic (or other 3D printer material) isleft exposed, making it clear to the viewer that this is a functionalsupport that is not part of the original image. Which approach ispreferred may be an artistic decision for the user.

It will also be appreciated that the pixel resolution of the renderedimages may be higher than the voxel resolution of the 3D printer at thescale chosen for printing. Consequently features of the renderedenvironment may sometimes have a high spatial frequency relative to thevoxel resolution of the 3D printer. Accordingly the images may befiltered to a compatible resolution before projection into the sharedvirtual model space, to remove such high-frequency detail and therebyavoid aliasing effects in the resulting model. In this case, wherereserved colour channel values are to be used by procedural rules, theseare preserved or reinserted into the lower resolution image afterfiltration. The depth information may be similarly filtered.

Hence, using any or the above techniques, or any other suitabletechniques, a property of the 3D surface geometry (such as its thicknessor connective elements) may be modified to form 3D printable model datathat may be output for 3D printing either immediately or prospectivelyin the future. Consequently, a direct product of modifying a property ofthe 3D surface geometry to form 3D printable model data that is thenoutput, is 3D printable model data. This 3D printable model data may beoutput directly to a printer, or alternatively to a local or remote,private or public repository for review by the user or others, prior toany decision to print the model.

Thickening Algorithm

The above example relating to FIGS. 10A-E discloses, for objects in thevirtual environment with zero thickness, a method of extruding theobject to achieve sufficient thickness for that object to beself-supporting when it is eventually 3D printed.

In the example of the wall illustrated in FIGS. 10 A-E, it is assumedthat only one side of the wall is textured (namely, the side facingtowards the navigable virtual environment), and so matters of textureand colour were not of concern for the extruded component.

However the situation is different where a texture (sometimes the same,and sometimes different) is visible on both sides of the zero thicknessobject.

A typical and frequent example of such an object is clothing for theuser's in game character, such as a cloak. Also hair is often renderedusing zero thickness structures. Other common zero thickness objectsinclude curtains, flags, rugs, paintings and other decorative elementsof the environment.

It will be appreciated that whilst such objects have zero thickness,they are rarely two-dimensional in the strict sense of only lying on asingle plane and having a single normal; typically the object willundulate and occupy three-dimensional space

Hence separately to the issue of a desired physical thickness, there isa potential difficulty in processing elements such as clothing and flagsthat have zero thickness but different textures/colours on each side.

For example where a scene is captured using the above-describedphotogrammetry techniques, and/or using point cloud capture, thisresults in capturing different colour values for coincident pointsdepending on the camera viewpoint (for example, a camera seeing one sideof the zero thickness object will capture a first point colour value,was a camera on the other side of the zero thickness object may capturea second different point colour value for the same position).

It is therefore necessary to process such points to choose the correctcolours of such an object for a physical representation.

In principle it is possible to address these issues using replacementsupplementary geometry that has been thickened as desired, as discussedpreviously herein. However this may not be possible to implement fortechnical (storage) or economic reasons. Furthermore, in some cases thezero thickness object may be generated procedurally itself, oranimated/warped using physics simulations (e.g. cloth/hair physics),making substitution impractical.

Hence it is preferable to modify the point cloud and thicken the zerothickness object as appropriate either in the point cloud or duringtranslation to voxels for printing.

However simple morphological image processes suitable for thickeningobjects, such as dilation, do not cope with having different andcontradictory textures at the same point in space; typically thisresults in either the texture on one side of the object being ignored,or an effectively random mixture of pixels from the two textures—forexample depending on the order of camera data chosen and/or the numberof cameras that captured data for a particular pixel, etc.

Accordingly, in an embodiment of the present invention, points in thepoint cloud are represented as 3D elements (e.g. voxels), as describedpreviously herein in relation to photogrammetry. Preferably the voxelshave a resolution corresponding to the 3D pixel (i.e. voxel) resolutionof the target printer, or a higher resolution that lends itself to cleaninterpolation by a printer driver, such as two or four timeshigher—although this is not essential. The resolution of the voxels maybe the same as or lower than the resolution of the point cloud. Thevoxel space can be thought of as similar to the point cloud space inthat it has a set of coordinates, optionally sharing an origin, and asnoted above optionally sharing a resolution, or more typically having alower resolution. A new voxel is generated when a point in the pointcloud is located in the notional volume occupied by a voxel. In practicethis may be done simply by mapping a flag indicating the existence of apoint in the point cloud from the coordinate system of the point cloudto the corner system of the voxel space, and then providing the newlyidentified voxel with relevant data, as described below.

Given such a voxel representation of the point cloud, it is possible tostore (if/as needed) colours for different faces of the voxel, orgenerally to store colours for points that are coincident in the pointcloud but have different colours at different normal (e.g.opposing/flipped/inverse normals, as on opposite sides of a zerothickness surface). It will be appreciated that as the point cloud hasthe same or higher resolution than the voxel representation,‘coincident’ can mean where two or more points exist within the volumeof a given voxel. Hence the purposes of the present invention, where theresolutions are different then ‘coincident’ is to the resolution of thevoxels and not to the points themselves.

Using this voxel representation, it will be appreciated that themajority of voxels will only need to store a single texture/colour valuebecause the corresponding points are part of an external surface of asolid virtual object, and the object does not comprise an additionalinternal texture. This will be true for example of the Blobman characterin FIG. 3 and the character captured using photogrammetry in FIG. 5.

By contrast, and referring to FIGS. 13A and 13B, a flag may havedifferent textures on either side whilst having a zero thickness. In thecase of the example figures, one face of the flag is predominantly blueand one is predominantly red.

Referring to FIG. 14A, consequently for a point on the flag a voxelneeds to represent different colours on different sides based on thedirection of the surface normal of the point being added from the flag.Hence the example voxel in FIG. 14A illustrates two points whosepositions are identical but which have different normals and colours(i.e. red facing in one direction and blue facing in the oppositedirection). It will be appreciated that the normal on the flag itselfmay be at an arbitrary vector, but this can be mapped to one of the sixfaces of the voxel cube for example by transposing the vector to anorigin at the centre of the cube and determining from which face thevector exits the cube. More simply, one may assume that the voxel cubesare rectilinearly arranged with respect to a world coordinate system andhence classify the normals to respective surfaces of the voxel cubeaccording to predetermined angular ranges.

In an embodiment of the present invention, when a first point is addedto a voxel, all the faces of the voxel take that colour. This simplyensures a texture/colour exists for all sides of the voxel, given thatin may not yet be known which sides of the voxel will be exposed to thesurface of the 3D printed model.

When subsequent coincident points are encountered and added, the colourof the face of the voxel corresponding to the normal of the subsequentpoint is changed to correspond to that of the point.

Hence referring now to FIG. 14B (which shows a notional plan view of avoxel and hence only four sides for illustrative purposes), if forexample if the first point added was a red point (a) then initially allsides of the voxel would be red. Subsequently if the next point was blue(b) then the face corresponding to the normal for that point wouldbecome blue. This would then continue for any additional coincidentpoints (c) until the point cloud capture process was complete. If thevoxel representation is of a lower resolution than the point cloudrepresentation, it is possible to have two points within the volume ofthe voxel whose normals both indicate the same face of the voxel; inthis case, an average of the colours for a given face may be calculated,or one of the colour values may be discarded.

Given the above voxel representation of coincident points in the pointcloud, it is then possible to thicken the object represented by thevoxels to a desired extent whilst preserving the colours of the sourceobject.

For the avoidance of doubt, in the event that the colours ‘red’ and‘blue’ are replaced with hatching in the figures, then the hatchingshown in FIG. 14B(a) corresponds to red, and the alternative hatching isblue.

It will be appreciated that for simple 3D printing services which relyon the printer material for colour (rather than using a subsequentpainting step) it will be necessary to thicken the model in order tophysically render the object, since a single 3D pixel of such a 3Dprinter cannot have two different colours as it is made from a singledeposition of material.

Hence for such printers, it is preferable to increase the thickness ofthe shape at least so that individual voxels of the object represent asurface of a single colour.

As noted above, most voxels will only have a single colour and so can beprocessed very efficiently, and the shape may only need to be adjustedfor reasons of physical strength/stability as discussed previouslyherein.

However, where a single voxel comprises two or more colours due tocapturing coincident points of a zero thickness object, the followingpropagation algorithm may be employed to thicken the structure so thateach voxel only uses one colour, whilst retaining the pattern of colourscaptured by the faces of the original voxel.

For a zero thickness object with two sides, initially represented at agiven position by a single voxel (as in FIG. 14A), then the simplestpropagation results in the object being represented at a given positionby two voxels, one for the colour of the object on each side, asillustrated in FIG. 15.

Accordingly, and more generally, a process for thickening the voxel in adesired direction may then proceed as illustrated in FIGS. 16A-B.

FIG. 16A shows a duplication process for the example voxel in FIG.14B(b), and hence is in plan view on the x-y plane, but the principle isidentical for propagation in the z-direction. The original voxel isduplicated to create a second adjacent voxel in a desired direction. Thedesired direction will be the one parallel to the dissimilar opposingfaces of the original voxel. In the illustrated plan-view example, theleft and right faces of the original voxel are different and so a newvoxel has been propagated on the X axis through both these faces asshown by the arrow.

Now, each voxel can be wholly assigned the colour corresponding to therespective opposing face of the original voxel on the propagationaccess. Hence the left-hand voxel adopts the colour of the left-handface of the original voxel whilst the right hand voxel adopts the colourof the right-hand face of the original voxel. More generally thereforethe first voxel and the duplicate voxel are respectively assigned thefirst and second colours according in an order along the first axiscorresponding to the order along a first axis of the two opposing facesof the first voxel having the first and second colours. Subsequently atleast the external faces of the voxels adopt their respective assignedcolour.

It is then possible to print both sides of the zero thickness objectusing the respective colours seen on each side of the object.

Whilst this addresses the simplest case for the simplest printingcapability, it will be appreciated that because a zero thickness objectmay undulate within three dimensions, and because a voxel may be at theedge of such an object (or may substantially form the object, in thecase of strands of hair and the like) then multiple surfaces of a voxelmay be visible in the final print and may have different colours. Hencemore generally, where it is desirable for a zero thickness (now onevoxel thick) object to be thickened further, for example for structuralstrength or due to colour printing limitations, then this may be donewhilst preserving the distribution of colours on the original voxel, asfollows:

Such a more general process for thickening the voxel in one or moredimensions may proceed as illustrated in FIGS. 17A-D. The illustrationis in plan view (x-y plane), but the principle is identical forpropagation in the z-direction.

FIG. 17A shows a duplication process for the example voxel in FIG.14B(c), which is the result of three coincident points having threedifferent normals.

Again, the original voxel is duplicated to create a second adjacentvoxel in a first desired direction parallel to dissimilar opposing facesof the original voxel. Again the voxel can simply be duplicated,preserving face colour information. Again in the illustration theduplication is on the x-axis. This time, however, initially the newvoxel changes the colour of the face adjacent the original voxel (i.e.face-to-face adjacent, not edge-to-edge adjacent) to match that of theadjacent face of the original voxel. Hence in the illustrated example,the adjacent face is changed from blue to red. In this way, the originalpattern on the external faces of the original voxel have been extendedin the x-direction whilst the internal faces of the resulting object arecolour consistent.

This process may be termed ‘pushing out’ a new voxel in the desireddirection.

The process may continue in another direction, whether becauseadditional thickness is required for structural reasons or for any otherreason.

Accordingly, duplication on the y-axis is shown in two steps in FIGS.17B and 17C.

For the purposes of example, the two voxels of FIG. 17A both havedifferent colour faces when viewed along the Y-axis. Duplicating both inthe Y-axis results in the arrangement shown in FIG. 17B. Subsequentlymatching the internal face colours to the colours of the adjacent facesof the original voxels is shown in FIG. 17C.

It will be appreciated that the four voxels of FIG. 17C now resemble afour times larger version of the original voxel of FIG. 14B(c), andhence the voxel and colour representation of the model has beenthickened.

It will be appreciated that by changing the notional x, y and z axes ofFIGS. 16 and 17, the same principle may apply for propagation in anyone, two or three of these axes, and in any order of axes. Indeed, theprocess can be applied simultaneously in two or typically three axes.

Finally referring now to FIG. 17D, once thickening is complete (as perFIG. 17C) then optionally a clean-up process may be applied in whichcolour information corresponding to internal faces of the resultingobject (i.e. any face of a voxel that is face-on adjacent to a face ofanother voxel) is deleted, so that only external faces retain colourinformation, as illustrated by FIG. 17D. Similarly optionally, voxelsthat have been thickened in this manner to create groups of adjacentvoxels may be replaced by larger cubic or rectilinear voxels.

The above thickening techniques are preferably implemented once pointcloud generation is complete, as this is when complete information isavailable about all the coincident points that may be found within thevolume of a given original voxel, and furthermore complete informationis also available about the position of all potential neighbouringvoxels. However alternatively the process can be implemented whenever asubsequent point appears within the original voxel volume that has acolour and/or normal that results in changing the colour of one of thefaces of the voxel cube. Clearly in the case of this latter approach,any subsequent point that appears within the volume of one of the newlygenerated voxels (which potentially may not correspond to/encompass anyexisting point at that stage I the point cloud generation process) willalso need to be treated as if it was a subsequent point for that voxel.

The above examples illustrate the case for thickening or pushing outfrom a single voxel; however typically voxels will not occur inisolation but will be part of a larger body, such as for example theflag of FIGS. 13A and B.

Hence whilst the basic principle remains the same, additional rules maybe imposed relating to selection of the preferred direction forthickening and the order in which to apply this process to voxels of theobject.

A first optional rule is that a voxel cannot be propagated into thespace occupied by another voxel. In principle this rule is moot becauseif one voxel is adjacent to another voxel, then one typically will notencounter a point in the point cloud having a normal that points towardsthe adjacent voxel, as this would indicate that a surface feature of theobject is pointing tangentially along the surface or even inwards.However it is possible, if the resolution of the point cloud issignificantly higher than the resolution of the voxels such that smallsurface features facing each other (such as for example corrugationsfrom a normal-mapped texture) are mapped to adjacent voxels so that thenormals point to the adjacent voxels, potentially causing the voxels topropagate into each other's space.

A second optional rule relating to prioritising the order of propagationis that where two or more adjacent voxels form a structure that is asingle voxel thick in at least one direction, the thickening processbegins at one or more edges of the structure and propagates inwards fromthe edge. A similar optional rule relating to prioritising the order ofpropagation is that where three or more adjacent voxels form a structurethat is a single voxel thick in at least one direction, the thickeningprocess begins with a voxel having a colour face whose colour is mostdifferent to the mean (average, median or mode) colour of the structure(for example, using RGB histograms or sum difference values from RGBmean values) and propagates away, the intention being to prioritisedistinctive features in the overall object.

It will be appreciated that in any case original voxels may comprise aflag indicating whether or not they have been processed by thethickening algorithm, and new voxels created by the process maysimilarly comprise a flag indicating that they should not be processedthemselves (and hence, depending on the implementation, this may simplybe the same flag, in this case falsely indicating that the new voxel hasbeen processed so that is not processed again). There may be a flag foreach propagation axis.

It will be appreciated that by use of the above techniques, a zerothickness object having different colours/textures on each side can be3D printed having sufficient thickness whilst retaining informationrelating to colours/textures on each side. It will be appreciated thatan object having zero thickness will typically result in a point cloudsurface that, within the corresponding voxel space, is one voxel layerthick, and where multiple points coincide within the volume of a singlevoxel their respective normals can be used to determine which face ofthe voxel is assigned the colour of the corresponding point.Subsequently, voxels can be propagated to a desired thickness using theabove techniques, namely by duplicating voxels along an axis of thevoxel that intersects different coloured faces of the voxel, thenoptionally homogenising the colour of internal faces, and/or optionallyremoving colour information for internal faces, and optionally makingthe propagation subject to rules that prevent one voxel from propagatinginto the space occupied by an existing voxel, and/or for prioritisingthe order of propagation.

Hence referring now to FIG. 18, a method of 3D print modellingcomprises:

In a first step 1810, generating a point cloud representation of atarget virtual object for 3D printing;

In a second step 1820, generating a voxel representation of the pointcloud by adding a respective first voxel when a respective first pointfrom the point cloud representation is located in the notional volumeoccupied by that first voxel;

In a third step 1830, assigning a first colour associated with the firstpoint to the faces of the first voxel; and if a second point from thepoint cloud representation is located within the volume of the firstvoxel,

In a fourth step 1840, assigning a second colour associated with thesecond point to a respective face of the new voxel corresponding to thenormal of the second point; and

In a fifth step 1850, thickening the voxel representation of the pointcloud by generating a duplicate voxel adjacent to the first voxel alonga first axis passing through two opposing faces of the first voxelrespectively having the first and second colours.

It will be apparent to a person skilled in the art that variations inthe above method corresponding to operation of the various embodimentsof the apparatus as described and claimed herein are considered withinthe scope of the present invention, including but not limited to:

-   -   target virtual object having at least one part having zero        thickness in at least one direction, and optionally the at least        one part of the target object having zero thickness has        different colours on each side at the same position;    -   changing the colour of the face of the duplicate voxel adjacent        to the first voxel to match that of the adjacent face of the        first voxel;    -   removing colour information for internal faces the resulting        plurality of voxels, internal faces being those faces face-on        adjacent to the face of a different voxel;    -   determining whether a duplicate voxel will occupy the same space        as an existing voxel, and if so, not performing the duplication;    -   the first voxel and the duplicate voxel being respectively        assigned the first and second colours according in an order        along the first axis corresponding to the order along a first        axis of the two opposing faces of the first voxel having the        first and second colours;    -   generating the point cloud representation of the virtual target        object by virtual photogrammetry using the renders of the target        virtual object, a direct product of which may be a voxel        representation of the point cloud in which at least one voxel        has been duplicated by the thickening step;    -   generating a 3D printable model data from the voxel        representation, and outputting the 3D printable model data for        3D printing, a direct product of which may be 3D printable model        data; and    -   3D printing a model based upon the output 3D printable model        data, a direct product of which may be a 3D printed model.        Object Selection

The above discussion has assumed that a target object(s) has beenselected from a region of the virtual environment; for example, the usermay select to just print their avatar, or their avatar and an opponent.

This selection may be done for example by a user interface for defininga bounding box around the desired object(s), or by a user interface forindicating specific objects within the game environment using a cursoror reticule.

Where the object(s) are selected in this manner in isolation from asupporting surface, a default surface such as a flat panel may begenerated having an area responsive to the size of the selectedobject(s). The selected object(s) can then be coupled to the supportsurface using the techniques described herein.

Editing

Once the printer geometry has been obtained using the above techniquesas applicable, then optionally they may be transferred to an editingapplication for the user to preview or modify the model before sendingit to the printer. For example it may be desired to review the model forpossible errors, and/or to edit the pose and positioning of certain keycharacters. The editor may likewise enable the addition of decals,either purely to painted textures, or to the surface structure ofobjects for example by deleting surface voxels so that the user caneffectively engrave messages into a surface of the model.

Final Print

Once the user is satisfied with their 3D model, it may be sent to theprinter. As described previously, a printer driver analyses the modeland divides it into layers of voxels for successive printing.

Where the printer is a domestic device either physically attached to theconsole or sharing a home network, then the printer driver may beimplemented on the entertainment device, and the resulting drawing listis sent to the printer by the entertainment device.

However optionally a printing service may be provided by theentertainment device's network (that is to say, a network provided forexample by the manufacturers of the entertainment device). This mayallow access to a more expensive and sophisticated 3D printer than theaverage consumer could afford, and consequently a better quality model.In these circumstances either the printer driver may still be on theconsole, to distribute processing load among users, or may be at acentral server connected to the 3D printer. In the first instance localprinter drivers will generate drawing lists that may be sent securely toa central print queue server, together with meta data relating to thepostal address of the user. In the second instance the entertainmentdevice securely sends the 3D model printer geometry to a central printerdriver that performs the relevant analysis to generate the requireddrawing lists before queueing them for printing. In either of thesecases, printing of the model may be contingent upon the payment of afee, for example via a payment card registered with the entertainmentdevice's network, or similarly may be contingent upon the receipt of avoucher which might be earned for example as a trophy or other in-gameaward, or as part of the purchase price of a game, entitling the user tothe creation of a predetermined number of 3D models from that game.

Hence a step of 3D printing a model based upon the output 3D printablemodel data directly results in a 3D printed model.

Waste Materials

It will be appreciated that 3D Printing techniques such as selectivelaser sintering can trap large quantities of the raw printer materialinside the resulting model. This is because such techniques applysuccessive layers of powder across the top of the model during printing,fusing only a small proportion corresponding to the model's peripherybut leaving the remaining powder inside that periphery untouched. Theoverall result is a volume of powder within which is a fused shell, withthe powder outside that shell being removable, whilst the powder insidethe shell is trapped.

This creates unnecessary costs in terms of wasted raw materials. It canalso affect the balance of the resulting objects. Consequently it iscommonplace to include vents or openings in models designed for 3DPrinting to allow the waste powder to be poured or washed out.

However, such vents or openings are not present in videogame charactersand are not desirable.

Accordingly, in an embodiment of the present invention, if apowder-based printer is to be used, a procedural rule is implemented tocalculate the effective volume of space within objects in the printedscene. Where that volume exceeds a threshold amount, the object istreated as a hollow body and a procedural rule creates an opening in thevoxel shell of the hollow body of a size that allows printer powder toflow out. Preferably, the opening is located on the underside of thehollow body and/or on a side facing away from the original viewpointdisplayed on screen to the user when they indicated that they wanted toprint the scene. The opening may be a hole in the voxel shell, orpreferably an annular gap forming a ring or some other shape of closedloop. In the case of an annular gap, this results in a loose cover inthe printed body that can be removed to allow the powder out, but whichcan then by adhered back onto the model, minimising the aesthetic impactof removing the powder on the finished object.

It will be appreciated that a 3D printed scene may comprise multiplesuch hollow bodies. For example, the scene may comprise the ground, acharacter such as Blobman, and a tree. The ground is typically an openshell. Meanwhile the tree rests upon it, with the ground forming thebase of the tree. Following calculations of the volume within the model,the tree trunk may be identified as a hollow object. Accordingly, a holemay be created in the ground beneath the tree, visible only if the modelis viewed from the underside. By contrast, the Blobman character issupported by connecting rods above the ground. Consequently if thecharacter torso is identified as a hollow object, an annular gap isincluded facing away from the original viewpoint, optionally at aposition a threshold distance from any connecting rod. This allows thetorso to be drained of powder but also to be repaired by affixing backin place the removable component of the torso created by the annulargap.

Optionally, the procedural rule may use features of the object itself toreduce the aesthetic impact of the hole or annular gap, for example bysetting some or all of one or more edges of the hole or gap to followadjacent a boundary between texture colours in a model, and/or to followadjacent a ridge or other discontinuity in the model surface.

The system may use the 3D model data to estimate the volume of printerraw material needed, and provide a quote estimating the cost of printingthe model before the user commits to doing so.

Referring back to FIG. 1, in an embodiment of the present invention adevice 10 (such the Sony Playstation 4) is arranged to generate a modelfor 3D printing, with the device comprising a point cloud generator(such as APU 20 operating under suitable software instruction) adaptedto generate a point cloud representation of a target virtual object for3D printing, a voxel generator (such as CPU 20A operating under suitablesoftware instruction) adapted to generate a voxel representation of thepoint cloud by adding a respective first voxel when a respective firstpoint from the point cloud representation is located in the notionalvolume occupied by that first voxel, a colour assignment processor (suchas APU 20 operating under suitable software instruction) adapted toassign a first colour associated with the first point to the faces ofthe first voxel, and a voxel thickening processor (such as APU 20operating under suitable software instruction), and if a second pointfrom the point cloud representation is located within the volume of thefirst voxel, the colour assignment processor is adapted to assign asecond colour associated with the second point to a respective face ofthe new voxel corresponding to the normal of the second point, and thevoxel thickening processor is adapted to thicken the voxelrepresentation of the point cloud by generating a duplicate voxeladjacent to the first voxel along a first axis passing through twoopposing faces of the first voxel respectively having the first andsecond colours.

Hence a general purpose computer operating under suitable softwareinstruction can implement the previously described method of thickeningvoxels where coincident points in a point cloud indicate differentcolours for different faces of a zero thickness virtual object.

Further, the device may comprise a colour adjustment processor (such asCPU 20A operating under suitable software instruction) adapted to changethe colour of the face of the duplicate voxel adjacent to the firstvoxel to match that of the adjacent face of the first voxel, and/or toremove colour information for internal faces the resulting plurality ofvoxels, internal faces being those faces face-on adjacent to the face ofa different voxel. Similarly the device may comprise a voxel overlapdetection processor (such as APU 20 operating under suitable softwareinstruction) adapted to determine whether a duplicate voxel will occupythe same space as an existing voxel, and if so, not performing theduplication. Similarly other processors, typically formed by the APU 20operating under suitable software instruction, may perform othercorresponding steps of the methods and techniques as described herein.

The additional techniques described herein may also be implemented bythe device 10 operating under suitable software instruction, and the 3Dprinting itself may be implemented by the device operating inconjunction with a 3D printer 100.

It will be appreciated that references to the APU and CPU above arepurely illustrative and that any suitable processor for a given role maybe used.

It will be appreciated from the above description that the methodsdescribed herein may be carried out on conventional hardware suitablyadapted as applicable by software instruction or by the inclusion orsubstitution of dedicated hardware.

Thus the required adaptation to existing parts of a conventionalequivalent device may be implemented in the form of a computer programproduct comprising processor implementable instructions stored on anon-transitory machine-readable medium such as a floppy disk, opticaldisk, hard disk, PROM, RAM, flash memory or any combination of these orother storage media, or realised in hardware as an ASIC (applicationspecific integrated circuit) or an FPGA (field programmable gate array)or other configurable circuit suitable to use in adapting theconventional equivalent device. Separately, such a computer program maybe transmitted via data signals on a network such as an Ethernet, awireless network, the Internet, or any combination of these or othernetworks.

The invention claimed is:
 1. A method of 3D print modelling, comprisingthe steps of: generating a point cloud representation of a targetvirtual object for 3D printing; generating a voxel representation of thepoint cloud by adding a respective first voxel when a respective firstpoint from the point cloud representation is located in the notionalvolume occupied by that first voxel; assigning a first colour associatedwith the first point to the faces of the first voxel; and if a secondpoint from the point cloud representation is located within the volumeof the first voxel, assigning a second colour associated with the secondpoint to a respective face of the new voxel corresponding to the normalof the second point; and thickening the voxel representation of thepoint cloud by generating a duplicate voxel adjacent to the first voxelalong a first axis passing through two opposing faces of the first voxelrespectively having the first and second colours.
 2. The method of claim1, in which the target virtual object has at least one part having zerothickness in at least one direction.
 3. The method of claim 2, in whichthe at least one part of the target object having zero thickness hasdifferent colours on each side at the same position.
 4. The method ofclaim 1, comprising the step of changing the colour of the face of theduplicate voxel adjacent to the first voxel to match that of theadjacent face of the first voxel.
 5. The method of claim 1, comprisingthe step of removing colour information for internal faces the resultingplurality of voxels, internal faces being those faces face-on adjacentto the face of a different voxel.
 6. The method of claim 1, comprisingthe step of determining whether a duplicate voxel will occupy the samespace as an existing voxel, and if so, not performing the duplication.7. The method of claim 1, in which the first voxel and the duplicatevoxel are respectively assigned the first and second colours accordingin an order along the first axis corresponding to the order along afirst axis of the two opposing faces of the first voxel having the firstand second colours.
 8. The method of claim 1, comprising the step of:generating the point cloud representation of the virtual target objectby virtual photogrammetry using the renders of the target virtualobject.
 9. The method of claim 1, being a voxel representation of thepoint cloud in which at least one voxel has been duplicated by thethickening step.
 10. The method of claim 7 comprising the steps of:generating a 3D printable model data from the voxel representation, andoutputting the 3D printable model data for 3D printing.
 11. The methodof claim 10, being 3D printable model data.
 12. The method of claim 10comprising the step of: 3D printing a model based upon the output 3Dprintable model data.
 13. The method of claim 11, being a 3D printedmodel.
 14. A non-transitory, computer readable storage medium havingcomputer executable instructions stored thereon, which when executed bya computer system, causes the computer system to perform 3D printmodelling by carrying out actions, comprising: generating a point cloudrepresentation of a target virtual object for 3D printing; generating avoxel representation of the point cloud by adding a respective firstvoxel when a respective first point from the point cloud representationis located in the notional volume occupied by that first voxel;assigning a first colour associated with the first point to the faces ofthe first voxel; and if a second point from the point cloudrepresentation is located within the volume of the first voxel,assigning a second colour associated with the second point to arespective face of the new voxel corresponding to the normal of thesecond point; and thickening the voxel representation of the point cloudby generating a duplicate voxel adjacent to the first voxel along afirst axis passing through two opposing faces of the first voxelrespectively having the first and second colours.
 15. A device arrangedto generate a model for 3D printing, comprising: a point cloud generatoradapted to generate a point cloud representation of a target virtualobject for 3D printing; a voxel generator adapted to generate a voxelrepresentation of the point cloud by adding a respective first voxelwhen a respective first point from the point cloud representation islocated in the notional volume occupied by that first voxel; a colourassignment processor adapted to assign a first colour associated withthe first point to the faces of the first voxel; and a voxel thickeningprocessor; and if a second point from the point cloud representation islocated within the volume of the first voxel, the colour assignmentprocessor is adapted to assign a second colour associated with thesecond point to a respective face of the new voxel corresponding to thenormal of the second point; and the voxel thickening processor isadapted to thicken the voxel representation of the point cloud bygenerating a duplicate voxel adjacent to the first voxel along a firstaxis passing through two opposing faces of the first voxel respectivelyhaving the first and second colours.